Despite the obvious benefits of using the metric system and SI units, the medical community persists with various types of bizarre systems of measurement.
One such system is the French catheter scale. To make matters even more confusing, it can be abbreviated in 5 ways:
- FR or Fr
- FG (meaning French Gauge)
- CH or Ch (meaning Charriére)
Joseph Charriére (1803-1876) was the inventor of the French catheter scale. He was a manufacturer of surgical instruments. He worked at a time when there was no standardised system of measurement – different industries and different countries could all do their own thing!
The basic principle of the French catheter scale is that the diameter of the catheter (in mm) is one-third the French size.
So a 10 FR catheter has a diameter of 3.33 mm. Note that this is the outer diameter – so catheters of the same FR size could have different lumen diameters depending on their wall thickness.
Here are 3 common anaesthetic examples:
(1) Y suction catheter
The manufacturers of this particular Y suction catheter have just labelled it as “14” without telling you that this is the FR size. The outer diameter is not stated on the packet, but we can work it out to be (14 / 3) = 4.6 mm.
(2) Intubating Bougie
For this particular bougie, the manufacturer helpfully tells you it’s 14 Fr, and the OD is 4.6 mm (we already worked this out in Example 1). The smallest ETT this bougie will go down is a 5.0 mm. (NB the sizes of ETT’s refer to their internal diameters).
(3) Double Lumen ETT
This manufacturer states the size as 37 Fr/Ch …. I wonder how many clinicians know that Fr and Ch are actually the same thing? They also state it is a 12.3 mm tube – but unfortunately, they don’t specify that this is the outer diameter (OD)*. Double lumen tubes change shape along their length, so 12.3 mm only applies to the OD at the widest point. The internal diameter of the tracheal and bronchial lumens are each different, and also change shape along the length of the tube!!
* the size in mm stated on a DLT refers to its outer diameter, but for all other ETT’s the size in mm refers to its inner diameter.
I’m sure you can think of other examples… how about urinary catheters, nasogastric tubes, intercostal catheters, and surgical drains!!
It would probably be impossible to change from this system, because it has become so ingrained. Even though few people know the true size of many of these tubes, they seem to know which one to use!